In natural gas processing plants, the gas is treated to remove acid gases, sulphur compounds and water in order to produce a gas which is suitable for distribution to the end-user. The removal of water vapour from natural gas is essential in order to avoid the formation of hydrocarbon hydrates in the gas stream and also to avoid the water condensing out of the gas in pipelines and process equipment which may then lead to corrosion problems. For this reason the maximum amount of water allowable in processed natural gas is regulated. Several methods of removing water to depress the dew point of natural gas are practised commercially in gas-processing plants. The methods employed include contacting the gas with a bed of a solid desiccant such as a silica gel or molecular sieve, and the use of liquid desiccant compounds. In the latter case, a typical process involves contacting the wet gas stream with a glycol solution in order to strip water from the gas stream into the glycol solution. This process is known as glycol dehydration and is very widely used in gas processing operations. The glycol solution is regenerated (i.e. dried) by removing the water in a boiler or in a flash-tank apparatus and the resulting dry glycol is recycled back to the drying process.
In some drying processes, methanol or another alcohol may be used as an alternative to glycol or as a mixture with a glycol. Methanol in particular, optionally admixed with a glycol, is often used to dehydrate gas flowing out of a sub-sea well, where the formation of methane or ethane hydrates is a problem due to the cooling of the gas as it exits the well. The methanol and/or glycol is introduced directly into the pipeline, usually by spraying, in order to absorb water in the gas stream. The wet methanol and/or glycol is then separated from the gas at a location downstream of the injection point, usually at the associated gas processing plant, and then regenerated by drying, e.g. by distillation before being returned to the injection point.
Mercury compounds are found in petroleum fluids such as natural gas. The concentration of mercury in natural gas is dependent on the source and may vary from very low, e.g. about 10 ngm−3 to high e.g. 5,000 ngm−3 or more. When natural gas containing mercury compounds is dried in a glycol dryer, we have found that the mercury compounds tend to partition into the glycol and thence into the water and dissolved gases separated from the glycol during glycol drying and regeneration, during which process desorbed gas, water and mercury may be evolved. The drying of the glycol for regeneration is usually done by heating to an elevated temperature which is above the boiling point of water. The temperature used is selected to be lower than the boiling point of the glycol in order to avoid decomposition of the glycol which may occur on prolonged heating. Therefore the temperature used for regeneration depends on the boiling point and thermal stability of the glycol used, e.g. triethylene glycol may be regenerated at about 200° C. On cooling, the water in vented gas is condensed and discharged. The waste-water and desorbed gases may contain significant amounts of mercury and thereby pose an environmental problem on discharge.
Alcohols, particularly methanol may be used in other purification processes, for example in the Rectisol™ wash process which is used to remove sulphur compounds and CO2 from gas streams, including hydrogen, ammonia or methanol syngas streams amongst others. Where the gas stream contains mercury or compounds of mercury, then the regeneration of the methanol wash liquid may risk discharge of the mercury to the atmosphere. Methanol absorbents may be regenerated by fractional distillation techniques in a conventional manner.